We're looking at rearends for the '50 3100, the 8.8 Explorer is offset,& info on therangerstation.com notes the Mustang 8.8 is centered, from 2005 uses 31 spline axles, doesn't need c-clip eliminators as the rotor-caliper-caliper mount will hold the axle into the housing.

What i haven't been able to find is the width of this axle, has anyone used or measured one?, i've Googled numerous sites with no luck, thanks in advance for any info.

try this site, it has some info on the ford diff. disregard some of it because it is about fitting into old maverics or falcons,but it does have some info on the mustang or explorer diff widths.
depending on how narrow you need the offset diff can be narrowed some by taking an offset unit, pulling the long tube out and replacing it with a short tube from another diff. that way you are still using a stock axle etc. they are held in with a pressed in plug apparently. you would want to make up a jig for the diff to sit in so you get everything dead on but that isn't too tough if you have some scrap laying around or if you are close to a scrap dealer who sells steel by the pound. you can take it back after you are done and gain some costs back.

an offset diff from a ranger may also work because they were narrower for some models. the offset isn't that big of a deal anyway unless it interferes with something already in place.

be aware that some specs are from the tire centre line not the wheel mounting flanges.

I cant tell you exactly how wide it is, at least not now - im out of town. But i can tell you it will work fine on your truck.

Here's one under my '55.2 Task Force truck. It is very close to the width of my former chevy 10 bolt and the stock rearend. I dont think i'm going this wide but the guy that is says I will be able to put a 315mm wide rear tire without tubbing the bed. I doubt I'll go bigger than a 295. Anyway, hope this helps:

These are about 59.5-60". Put one in my 55 chevy sedan and it works great!. I swapped the axles from side to side (cutting and welding involved) to center the pinion a little better, but with my 3100 is not an issue. They are cheap, strong, and readliy available. 373, limitied slip,and disc breaks. I don't know why you would need a c-clip elimination kit. I know it happens, but can't say I have ever seen an explorer on the side of the road with an axle missing. The axles are strong and unless you are going to put 600 HP to it, I don't think I would worry about it

The Explorer axles are easier to find and typically much cheaper. The offset of the Explorer axle should only be an issue if you have a triangulated 4 link. It should be fine with a parallel 4 link, leaf spring, or trailing arm rear suspension.

Not all of the Mustangs have an 8.8 and I don't think you will have as many gear ratio options either.

on my 8.8 I used and old brake rotor flange for a pattern. cut off the main part of the rotor so just the wheel mounting area left. I took it to a machinist buddy who drilled it for a chevy pattern between the ford pattern (5 lug) just 1/8 inch holes. you could do this yourself probably. then brought that home and bolted it up with the existing ford lugs over the existing ford rotor, like mounting a wheel. then drilled through the predrilled holes to mark the rotor, removed and drilled the original brake rotors on a drill press. do over again to mark the axles, which you have to hand drill while on the vehicle unless the diff is apart. then punch out the ford studs and drill the parts the proper size you need for the new studs. remember to allow a press fit on the new studs in the axle but a loose fit through the brake rotors. the pattern can hang on the wall or be kept with the truck in case you blow something up 100 miles from home. get studs with nuts and thread to match the front wheels.

The Explorer rear axle is 59.5", which was too narrow for me. So I used 1" thick wheel spacer/adapters to take care of the bolt pattern issue.

I know some people don't trust wheel spacers, but I got some hub centric spacers that fit perfectly over the Explorer flanges and and are also an exact fit for the center bore of my wheels. My wheels are 20x8.5 and I believe the backspace is 4.75, the tire sits perfect between the bedside and fender edge.

Basically you could use the adapters to dial in your width if the axle is too narrow.

The Explorer rear axle is 59.5", which was too narrow for me. So I used 1" thick wheel spacer/adapters to take care of the bolt pattern issue.

I know some people don't trust wheel spacers, but I got some hub centric spacers that fit perfectly over the Explorer flanges and and are also an exact fit for the center bore of my wheels. My wheels are 20x8.5 and I believe the backspace is 4.75, the tire sits perfect between the bedside and fender edge.

Basically you could use the adapters to dial in your width if the axle is too narrow.

I have spacers on mine to. Hub centric billet aluminum. They are a tad wide now since I bought 1.5" ones. Will wait till I get my wheels to determine whether to get some one inch ones. Not too expensive. My ole 235 is not going to make enough HP for it to be a big concern